British naturalist, born at Usk, in Monmouthshire, on the 8th of January 1823. After leaving school he assisted an elder brother in his work as a land surveyor and architect, visiting various parts of England and Wales. Living in South Wales, about 1840 he began to take an interest in botany, and began the formation of a herbarium. In 1847 he took his first journey out of England, spending a week in Paris with his brother and sister.

In 1844-45, while an English master in the Collegiate School at Leicester, he made the acquaintance of H. W. Bates, through whose influence he became a beetle collector, and with whom he started in 1848 on an expedition to the Amazon. In about a year the two naturalists separated, and each wrote an account of his travels and observations. Wallace's Travels on the Amazon and Rio Negro was published in 1853, a year in which he went for a fortnight's walking tour in Switzerland with an old schoolfellow.

On his voyage home from South America the ship was burnt and all his collections lost, except those which he had despatched beforehand. After spending a year and a half in England, during which time, besides his book on the Amazon, he published a small volume on the Palm Trees of the Amazon, he started for the Malay Archipelago, exploring, observing and collecting from 1854 to 1862. He visited Sumatra, Java, Borneo, Celebes, the Moluccas, Timor, New Guinea and the Aru and Ké Islands. His deeply interesting narrative, The Malay Archipelago, appeared in 1869, and he also published many important papers through the London scientific societies.

The chief parts of his vast insect collections became the property of the late W. W. Saunders, but subsequently some of the most important groups passed into the Hope Collection of the university of Oxford and the British Museum. He discovered that the Malay Archipelago was divided into a western group of islands, which in their zoological affinities are Oriental, and an eastern, which are Australian. The Oriental Borneo and Bali are respectively divided from Celebes and Lombok by a narrow belt of sea known as "Wallace's Line", on the opposite sides of which the indigenous mammalia are as widely divergent as in any two parts of the world. Wallace became convinced of the truth of evolution, and originated the theory of natural selection during these travels.

In February 1855, staying at Sarawak, in Borneo, he wrote an essay "On the Law which has regulated the Introduction of New Species" (Ann. and Mag. Nat. Hist., 1855). He states the law as follows: "Every species has come into existence coincident both in time and space with a pre-existing closely allied species." He justly claims that such a law connected and explained a vast number of independent facts. It was, in fact, a cautious statement of a belief in evolution, and for three years from the time that he wrote the essay he tells us that "the question of how changes of species could have been brought about was rarely out of my mind."

Finally, in February 1858, when he was lying muffled in blankets in the cold fit of a severe attack of intermittent fever at Ternate, in the Moluccas, he began to think of Thomas Malthus's Essay on Population, and, to use his own words, "there suddenly flashed upon me the idea of the survival of the fittest." The theory was thought out during the rest of the ague fit, drafted the same evening, written out in full in the two succeeding evenings, and sent to Charles Darwin by the next post. Darwin in England at once recognized his own theory in the manuscript essay sent by the young and almost unknown naturalist in the tropics, then a stranger to him. "I never saw a more striking coincidence", he wrote to Charles Lyell on the very day, on the 18th of June, when he received the paper: "if Wallace had my manuscript sketch written out in 1842, he could not have made a better short abstract! Even his terms now stand as heads of my chapters."

Under the advice of Sir Charles Lyell and Joseph Dalton Hooker, the essay was read, together with an abstract of Darwin's own views, as a joint paper at the Linnean Society on the 1st of July 1858. The title of Wallace's section was "On the Tendency of Varieties to depart indefinitely from the Original Type." The "struggle for existence", the rate of multiplication of animals, and the dependence of their average numbers upon food supply are very clearly demonstrated, and the following conclusion was reached: "Those that prolong their existence can only be the most perfect in health and vigor... the weakest and least perfectly organized must always succumb."

The difference between Lamarck's theory and natural selection is very clearly pointed out. "The powerful retractile talons of the falcon and the cat tribes have not been produced or increased by the volition of those animals; but among the different varieties which occurred in the earlier and less highly organized forms of these groups, those always survived longest which had the greatest facilities for seizing their prey. Neither did the giraffe acquire its long neck by desiring to reach the foliage of more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them."

With such clear statements as these in the paper of the 1st of July 1858, it is remarkable that even well-known naturalists should have failed to comprehend the difference between Lamarck's and the Darwin-Wallace theory. Wallace also alluded to the resemblance of animals, and more especially of insects, to their surroundings, and points out that "those races having colors best adapted to concealment from their enemies would inevitably survive the longest." In 1871 Wallace's two essays, written at Sarawak and Ternate, were published with others as a volume, Contributions to the Theory of Natural Selection.

Probably, next to the Origin of Species, no single work has done so much to promote clear understanding of natural selection and confidence in its truth; for in addition to these two historic essays, there are others in which the new theory is applied to the interpretation of certain classes of facts. Thus one treats of "Mimicry" in animals, another on "Instinct", another on "Birds' Nests." Each of these served as an example of what might be achieved in the light of the new doctrine, which, taught in this way and in an admirably lucid style, was easily absorbed by many who found the more complete exposition in the Origin very hard to absorb. In this work, and in many of his subsequent publications, Wallace differs from Darwin on certain points. Thus the two concluding essays contend that man has not, like the other animals, been produced by the unaided operation of natural selection, but that other forces have also been in operation. We here see the influence of his convictions on the subject of "spiritualism."

More recently he expressed his dissatisfaction with the hypothesis of "sexual selection" by which Darwin sought to explain the conspicuous characters which are displayed during the courtship of animals. The expression of his opinion on both these points of divergence from Darwin will be found in Darwinism (1889), a most valuable and lucid exposition of natural selection, as suited to the later period at which it appeared as the Essays were to the ealier. Darwin died some years before the controversy upon the possibility of the hereditary transmission of acquired characters arose over the writings of Weismann, but Wallace has freely accepted the general results of the German zoologist's teaching, and in Darwinism has presented a complete theory of the causes of evolution unmixed with any trace of Lamarck's use or disuse of inheritance, or Buffon's hereditary effect of the direct influence of surroundings. Tropical Nature and other Essays appeared in 1878, since republished combined with the 1871 Essays, of which it formed the natural continuation. One of the greatest of his publications was the Geographical Distribution of Animals (1876), a monumental work, which every student will maintain fully justifies its author's hope that it may bear "a similar relation to the eleventh and twelfth chapters of the Origin of Species as Mr. Darwin's Animals and Plants under Domestication bears to the first." Island Life, which may be regarded as a valuable supplement to the last-named work, appeared in 1880.

Turning to his other writings, Wallace published Miracles and Modern Spiritualism in 1881. Here is given an account of the reasons which induced him to accept beliefs which are shared by so small a proportion of scientific men. These reasons are purely experimental, and in no way connected with Christianity, for he had long before given up all belief in revealed religion. In 1882 he published Land Nationalization, in which he argued the necessity of state ownership of land, a principle which he had originated long before the appearance of Henry George's work. In Forty-five Years of Registration Statistics (1885) he maintained that vaccination is useless and dangerous. Wallace also published an account of what he held to be the greatest discoveries as well as the failures of the 19th century, The Wonderful Century (1899).

His later works include Studies, Scientific and Social (1900), Man's Place in the Universe (1903) and his Autobiography (1905). Possessed of a bold and intensely original mind, his activities radiated in many directions, apparently rather attracted than repelled by the unpopularity of a subject. A non-theological Athanasius contra mundum, he has the truest missionary spirit, an intense faith which would seek to move the mountains of apathy and active opposition. Whatever may be the future history of his other views, he will always be remembered as an originator of a principle more illuminating than any which has appeared since the days of Isaac Newton, as one of its two discoverers whose scientific rivalry was only the beginning of a warm and unbroken friendship.

Wallace was married in 1866 to the eldest daughter of the botanist, William Mitten, of Hurstpierpoint, Sussex. In 1871 he built a house at Grays, Essex, in an old chalk-pit, and after living there four years, moved successively to Dorking (two years) and Croydon (three years). In 1880 he built a cottage at Godalming near the Charterhouse school, and grew nearly 1000 species of plants in the garden which he made. In 1889 he moved to Dorsetshire. After his return to England in 1862 Wallace visited the continent, especially Switzerland, for rest and change (1866 and 1896) and the study of botany and glacial phenomena (August 1895). He also visited Spa, in Belgium, about 1870, and in October 1887 went for a lecturing tour in the United States. He delivered a course of six Lowell lectures in Boston, and visited New York, New Haven, Baltimore, etc., spending the winter at Washington. The following March he went to Canada and Niagara, and then made his way westwards. He saw the Yosemite Valley, the Big Trees, and botanized in the Sierra Nevada and at Gray's Peak. In July he returned to Liverpool by way of Chicago and the St. Lawrence.

The first Darwin medal of the Royal Society was awarded to Wallace in 1890, and he had received the Royal Medal in 1868. A pension was awarded him by Gladstone at the beginning of 1881. He received the degree of D.C.L. from Oxford in 1889, and of LL.D. from the university of Dublin in 1882. He was president of the Entomological Society of London in 1870-71.